VT Sigma Profile Databases

Thermodynamic methods based on conductor-like screening models (COSMO) originated from the use of solvation thermodynamics and computational quantum mechanics. These methods rely on sigma profiles specific to each molecule. A sigma profile is a molecular-specific distribution of the surface-charge density, which enables the application of solvation-thermodynamic models to predict vapor-liquid and solid-liquid equilibria, and other properties. Two COSMO-based thermodynamic models are COSMO-RS (realistic solvation) developed by Klamt and his colleagues, and COSMO-SAC (segment activity coefficient) published by Lin and Sandler. Quantum mechanical calculations for generating the sigma profiles represent the most time-consuming and computationally expensive aspect of using COSMO-based methods. A growing number of scientists and engineers are interested in the COSMO-based thermodynamic models but are intimidated by the complexity of performing quantum mechanical calculations. We present two open literature databases, the VT-2005 Sigma Profile Database and the VT-2006 Solute Sigma Profile Database, that contain sigma profiles for 1,645 unique compounds. We design both of these databases for use with the conductor-like screening model ? segment activity coefficient (COSMO-SAC), a liquid-phase activity-coefficient model. The databases contain the necessary information to perform binary and multicomponent VLE and SLE predictions.

VT-2005 Sigma Profile Database

The VT-2005 Sigma Profile Database contains 1432 compounds, the majority of which are solvents and small molecules. We validate the VT-2005 Sigma Profile Database by pure component vapor pressure predictions. This work also studies the effects of conformational isomerism on VLE and SLE property prediction for a select few cases. These conformations are denoted by their index number and appropriate letter.

VT-2006 Solute Sigma Profile Database

The VT-2006 Solute Sigma Profile Database consists 206 of larger, pharmaceutical-related solutes and 32 solvents. We validate the VT-2006 Solute Sigma Profile Database by solid solubility predictions in pure and mixed solvents compared with literature data from multiple sources. This work also studies the effects of conformational isomerism SLE property prediction. These conformations are denoted by their index number and appropriate letter.

Sigma Profile Summary

COSMO models require sigma profiles in a manner similar to the way UNIFAC requires parameter databases, with one exception: sigma profiles are molecule-specific whereas UNIFAC parameters are functional group-specific. Furthermore, UNIFAC requires binary interaction parameters for the functional group pairs (binary interactions between groups). As a result, a sigma-profile database is much smaller and easier to use than the UNIFAC parameter database. Sigma profiles require only the molecular structure and a density-functional theory calculation to incorporate new compounds into the database.

The COSMO model creates a cavity around a solute molecule and then ideally dissolves it into an infinite conducting medium. We summarize this process with this figure:

We calculate the surface charge distribution or the SIGMA PROFILE. The sigma profile shows the probability of a surface segment having a specific charge density that determines the interaction energy with other surface segments in the liquid phase.

Resources

This work also provides a FORTRAN program to generate additional sigma profiles, as well as a FORTRAN program to generate binary and multicomponent phase-equilibrium (VLE and SLE) predictions using the COSMO-SAC model. Our sigma profile database will facilitate predictions of thermodynamic properties and phase behaviors from COSMO-based thermodynamic models. We offer detailed tutorials and procedures for use with our programs so the reader may also use their own research as well.